Field of the Invention
The present invention relates to stents; more particularly, this invention relates to stents for treating vessels of the body.
Description of the State of the Art
Radially expandable endoprostheses are artificial devices adapted to be implanted in an anatomical lumen. An “anatomical lumen” refers to a cavity, duct, of a tubular organ such as a blood vessel, urinary tract, and bile duct. Stents are examples of endoprostheses that are generally cylindrical in shape and function to hold open and sometimes expand a segment of an anatomical lumen (one example of a stent is found in U.S. Pat. No. 6,066,167 to Lau et al). Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce the walls of the blood vessel and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through an anatomical lumen to a desired treatment site, such as a lesion. “Deployment” corresponds to expansion of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into an anatomical lumen, advancing the catheter in the anatomical lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon prior to insertion in an anatomical lumen. Crimping refers to an iris-type or other form of crimper, such as the crimping machine disclosed and illustrated in US 2012/0042501. At the treatment site within the lumen, the stent is expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn from the stent and the lumen, leaving the stent at the treatment site. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath. When the stent is at the treatment site, the sheath may be withdrawn which allows the stent to self-expand.
The stent must be able to satisfy a number of basic, functional requirements. The stent must be capable of withstanding the structural loads, for example, radial compressive forces, imposed on the stent as it supports the walls of a vessel after deployment. Therefore, a stent must possess adequate radial strength. After deployment, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it. In particular, the stent must adequately maintain a vessel at a prescribed diameter for a desired treatment time despite these forces. The treatment time may correspond to the time required for the vessel walls to remodel, after which the stent is no longer necessary for the vessel to maintain a desired diameter.
A compliance-matching stent structure exhibits a high degree of radial flexibility at the end rings of the stent. These relatively ‘high compliance’ end-ring structures allow the deployed stent to better match the compliance of the adjacent vessel wall, thereby providing for improved pulsatile hemodynamics. In order to also provide sufficient scaffolding support against compressed plaque, the stent structure is relatively stiff in the middle-section of the stent. An example of a compliance-matching stent is described in U.S. Pat. No. 6,206,910. While the structure described in this publication aims to utilize low end-ring radial stiffness to enable improved hemodynamics, it may not adequately scaffold plaques acutely, which is often the primary function of a deployed stent. Calcified lesions, ostial lesions, and total occlusions all require a high degree of radial stiffness to avoid localized collapse after deployment. Against these 3 challenges and others, the structure disclosed in U.S. Pat. No. 6,206,910, may not perform adequately. For example, when stenting a lesion with even a modest plaque volume, longitudinal plaque migration is known to occur. In these cases the stent should have acute radial stiffness sufficient at the ends of the deployed structure during an acute period, i.e., within the first 1-2 weeks, or month following implantation. At later timeframes after deployment, however, a stented vessel will remodel positively over time, thereby requiring less radial stiffness. For this reason, a stent structure is desired that provides high end-ring radial stiffness acutely, and low end-ring radial stiffness in the long term after vascular remodeling occurs.
There is a need for improving long term vascular healing following stent implantation without adversely affecting stent stiffness characteristics needed during an acute period following implantation. Such long-term healing has been inhibited by a stent structure no longer needed to provide vascular support or where less support is needed from the stent. However, this support is needed during the acute period. It is desirable therefore to improve upon a stent having a variable radial stiffness and/or variable longitudinal or bending stiffness so that the stiffness reduces after a predetermined amount of time has elapsed following implantation.